Geminid meteor shower peaks

The peak of the annual Geminid meteor shower delivered over 100 shooting stars per hour in the early hours of 14 December. Observers in California were the first to report and others around the world are expected to have seen a good show.

Unlike the Leonids, the shower's maximum intensity is spread over nearly 24 hours, meaning observers everywhere had a chance of seeing the shooting stars. Estimates of the number of fiery streaks crossing the sky were accurate.

A 1998 Geminid photographed by Yukihiro Kida in Japan

The fragments which burn up in the Earth's atmosphere travel at 126,000 km/h and come from a mysterious object called 3200 Phaethon.

Normally, meteor showers result from dust which has been blown away from comets which have passed near the Sun. The warmth of our star melts part of the icy comet, releasing rock fragments.

But 3200 Phaethon, discovered in 1983 by Nasa's Infrared Astronomical Satellite, is apparently a rocky asteroid. Its orbital match with the Geminid meteors is so close, however, that it must be the source of the meteors.

Estimates of the density of the Geminid meteoroids are between one and two grams per cubic centimetre. This is less dense than typical asteroid material (three gm/cc), but several times greater than cometary dust flakes (0.3 gm/cc).

These hybrid characteristics have led many astronomers to believe that Phaethon is an extinct or dormant comet that has accumulated a thick crust of interplanetary dust grains. Phaethon's thick mantle gives it the outward appearance of an asteroid, but underneath lies the nucleus of a comet.

Watching the Geminids

Although the peak of the meteor shower has passed, some activity will still be visible until 17 December.

Geminid meteors appear to emanate from the constellation Gemini and can be seen any time after 2200 local time, when the constellation is well above the horizon. The rate increases greatly after midnight when the local sky is pointing directly into the Geminid meteoroid stream. The best time to look is during the early morning hours between about 0200 and dawn.

To find the constellation Gemini at 0200 (from the northern hemisphere), go outside and face south. It will appear approximately 45 to 60 degrees above the horizon. Earlier in the evening, from 2200 until midnight, Gemini will be about 30 degrees above the horizon in the southeast.

The naked eye is usually best for seeing meteors, which can often streak more than 45 degrees across the sky. The field of view of most binoculars and telescopes is simply too narrow.

If you are watching in the Northern Hemisphere, dress warmly as the nights are likely to be cold.